With the increased emphasis on energy conservation over the last 20 years, buildings are being constructed with more insulation and tighter construction techniques, thus reducing natural ventilation to the building. This decrease in natural ventilation has resulted in less fresh air for occupants of a building leading to what is called "sick building syndrome". In response to this problem, building standards have been changed to require controlled ventilation in adequate amounts to insure good "indoor air quality" a phrase which has recently become quite a buzzword in the heating, ventilation, and air conditioning industry. An example of this change is in ASHRAE Standard 62-1989 which has increased the ventilation requirements for schools to 15 CFM per student. Most standards previously called for 5 CFM per student. This has caused a large increase in a structure's HVAC load thus requiring larger, noisier and more expensive systems at significantly higher operating costs. In certain climatic regions, this also increases the latent (moisture removal) load of the building beyond the capability of conventional HVAC systems resulting in very high and uncontrolled humidity inside the building.
Prior HVAC systems employed different methods of varying sophistication to control the air which is conditioned and circulated within a structure. The control means used often depends on the type of structure for which ventilation is required as well as structure location. Temperature, humidity, and minimum outside air are three typical quantities which need be controlled. As previously mentioned, state statutes, building codes, ASHRAE standards, and the like often require that schools and other buildings provide minimum amounts of outside air. These facilities must use an HVAC system which can meet the necessary requirements. Frequently, a structure will require increased or decreased amounts of outdoor air when its use changes. A new or completely retrofitted HVAC system is then required to meet those new requirements.
As an example, portable classrooms have become popular in some parts of the United States where enrollment size shifts to various locations within a district. To meet the space requirements needed for such an enrollment flux, portable classrooms are moved from location to location. The HVAC unit attached to the portable classroom may be inadequate for the environmental conditions in the new location, thus requiring either a new HVAC system or a complete retrofit of the old system. A new HVAC system or retrofit can be expensive. For example, as codes and standards for indoor air quality change, an HVAC system must either be retrofitted or replaced to meet the new requirements.
Prior to the invention disclosed in U.S. application Ser. No. 016,153, existing HVAC systems could not be easily retrofitted with a new components and did not have modular ventilation units which were easily interchangeable. Methods or systems which could readily adapt to a structure's changed needs did not exist. Further, no systems were available which provided a heat recovery device as a built-in item in an air conditioner, heat pump or gas/electric type wall mounted heating and cooling system. The invention described in the above-mentioned application addressed the above concerns.